1. Technical Field
This invention relates generally to a method for processing exhaust gases discharged from lean-burn combustion systems, and more particularly to such a method in which products of combustion discharged from lean-burn combustion systems are discharged as products of stoichiometric combustion.
2. Background Art
The efficiency, power, and emissions characteristics of modern, reciprocating engines are a very strong function of the combustion system. There are two primary combustion systems in common use. Of these, the most common is the spark-ignited (SI) Otto-cycle engine, which derives its output power from the combustion of a pre-mixed, fuel-air-diluent charge by a propagating flame within the combustion chamber. In Si combustion systems, the balancing act of air and fuel is important, because its combustion occurs ideally at a single particular air/fuel ratio, the stoichiometric one of about 14:1 by weight. It can be finessed to run leaner, i.e., in lean- burn SI regimes of perhaps 30:1, but not without the complexities of direct injection and other tradeoffs. Spark-ignition engines generally suffer from low thermal efficiencies at light-to-part load, due to the necessity of throttling the airflow through the engine, to provide a means of load control. Additionally, the full-load efficiency and power of these engines suffers due to engine design and control limitations brought about by the possibility of high-load knock, or autoignition of the combustible gases within the combustion chamber. The compression ratio of these engines is lower than the optimum value for efficiency to avoid the knock problem. Additionally, the ignition timing for the combustion process is retarded from optimal values for efficiency to avoid knock and reduce NOx emissions. Increases in the efficiency of these engines have been accomplished utilizing lean-burn strategies with turbocharging. However, the knock problem persists and continues to limit the maximum efficiency of these engines.
Additionally, exhaust NOx reduction strategies, such as timing retardation, exhaust gas recirculation, lean NOx catalysts in selective catalytic reduction lead to further reductions in overall engine efficiency.
The second, predominant, conventional combustion system utilizes the diesel-cycle, which derives its power from compression ignition (CI) and diffusion burning of the fuel spray injected directly into a mixture of air and diluent gases. In controlling output from full power to idle, a CI combustion system continues to ignite at air/fuel mixtures of 100:1 in leaner, contributing to a diesel engine's light-load efficiency. Although diesel engines do not suffer from knock, a problem of SI combustion systems, the maximum fuel-to-air ratio is limited by the production of exhaust particulates. Additionally, because the diesel combustion flame spreads at nearly stoichiometric proportions, NOx production is high. Exhaust gas recirculation and late injection timing have been used to control in-cylinder NOx formation, but future NOx regulations may require additional Nox reduction strategies, such as selective catalytic reduction or use of a lean-NOx catalyst. Legal restrictions of exhaust gas particulate levels generally require particulate aftertreatment devices, such as traps or particulate filters.
Lean-burn gasoline and Diesel engines offer the benefits of higher thermal efficiency, but suffer from difficulty with NOx emissions. Nitrogen is present in the air we breathe, and in the air that an engine consumes. Nitrogen does not burn, but it can oxidize at temperatures over 2500° F. Therefore, NOx formation is a problem associated with lean-burn combustion systems, both spark-ignited and compression ignition systems. Currently, the most promising technology for NOx reduction in lean-burn combustion systems is the use of a “Lean NOx Trap” (LNT) or a 3-way catalyst to reduce NOx while oxidizing unburned hydrocarbons. However, Three Way Catalysts require a continuous flow of stoichiometric combustion products and Lean NOx Traps require that products of stoichiometric combustion be passed through the catalyst periodically in order to regenerate the NOx trapping cites and convert the released NOx into N2 and CO2.
U.S. Pat. No. 5,749,334 granted May 12, 1998 to Hideyuki Oda, et al. for a Control System and Method for In-Cylinder Injection Internal Combustion Engine, is addressed to overcoming problems associated with lean-burn combustion systems. Oda, et al. uses control of fuel injection, ignition, and exhaust gas recirculation (EGR) rate to promote stable combustion in the engine. However, Oda does not provide a way to assure that the combustion products discharged as engine exhaust gases from the lean-burn combustion system, are products of stoichiometric combustion.
The present invention is directed to overcoming the problems associated with NOx production in lean-burn combustion systems. It is desirable to have a highly efficient in-cylinder method for processing the exhaust gases from lean-burn combustion systems in such a way that the processed gases exit the engine as products of stoichiometric combustion, which can subsequently be passed through a 3-way catalyst to reduce NOx while oxidizing unburned hydrocarbons, periodically used to regenerate a lean NOx adsorber.
The present invention advantageously provides a method to use excess air from previous engine cycles in a reburning process, within the combustion chamber, thus providing products of stoichiometric combustion for expulsion from the engine or for use by an emissions aftertreatment system.